1. Field of the Invention
This disclosure generally relates to electrical power systems, and more particularly to power system architectures suitable for controlling over voltages on a DC power bus system.
2. Description of the Related Art
An alternating current (AC) electric machine may be coupled to a direct current (DC) system via a bi-directional power converter device. When the AC electrical machine is operating as a motor, the converter provides power to the AC electric machine by inverting DC power, received from a DC power source, into AC power. This AC power is delivered to the AC electric machine. Furthermore, the converter may control various operating aspects of the AC electric machine, such as speed and/or torque.
Alternatively, the AC electric machine may operate as a generator when torque is applied to the machine shaft. For example, when an AC electric machine driven motor vehicle is braking, running by inertia, or running downhill, the converter provides DC power to the DC power source by rectifying AC power, generated by the AC electric machine, into DC power. This mode of operation is sometimes referred to as the regenerative mode since the generated power is stored back into the vehicle's power system.
The DC side of the bi-directional power converter may be coupled to a high voltage (HV) DC bus. Other components are also typically coupled to the HVDC bus, including the DC power source. The DC power source and other components are designed to operate up to a maximum DC voltage on the HVDC bus. In the event that the DC voltage on the HVDC bus exceeds the maximum DC voltage, referred to hereinafter as an “over voltage” condition, the DC power source and/or other components may become damaged.
A system controller is typically employed to control operation of the power converter such that voltage on the HVDC bus is regulated (controlled). One of the functions of the system controller is to prevent over voltage conditions on the HVDC bus when the AC electric machine is operating in a generator mode.
The system controller requires a power source. In the event that power is lost to the system controller, voltage on the HVDC bus cannot be properly regulated via the power converter because the system controller becomes inoperable. That is, under some operating conditions, undesirable over voltages may occur on the HVDC bus.